1. Field of the Invention
The present invention relates to an optical information recording medium having at least two optical information recording layers and particularly relates to a large-capacity optical information recording medium having dielectric multilayer reflecting layers having variable reflectance, an optical information recording medium manufacturing method adapted for formation of the dielectric multilayer reflecting layers and an optical information recording device adapted for recording/reproduction of information in/from the medium.
2. Description of the Related Art
Change of the refractive index of an optical information recording medium has been disclosed in Hiroki Yamamoto et al., “Nonlinear change of refractive index of Co3O4 thin films induced by semiconductor laser (λ=405 nm) irradiation”, Applied Physics Letters, volume 81, number 61, pp. 999-1001.
Construction of a communication system allowing high-speed communication of a large amount of information has been demanded with the recent development of information society using optical communication. An optical information recording medium for storing a large amount of optical information exemplifies an optical device essential for development of such large-volume high-speed optical communication. With the advance of digitalization of video information such as television pictures and improvement in image quality such as high definition, development of a large-capacity optical information recording medium allowing long-time recording while maintaining the high image quality state has been demanded urgently.
For the time being, a DVD having a capacity of 4.7 GB on single side has been put into practical use as an optical information recording medium and has been widespread as a medium for storing a large amount of animation information such as video, in addition to use in a computer. The DVD has been put into practical use not only as a read-only ROM (DVD-ROM) having a substrate in which information has been already written directly, but also as a rewritable recording/reproduction medium.
Development has been made in order to improve recording density of this type optical information recording medium. A laser beam with a wavelength of 650 nm shorter than the laser (780 nm) used for a CD or the like has been used as a means for attaining high density recording of information. In order to handle a large amount of information for computer graphics, digital high definition video, etc., it is however necessary to attain recording density increased by 4 times or 5 times as high as the recording density obtained by the 650 nm laser beam. In order to attain such high recording density, an optical disk using a blue semiconductor laser with a shorter wavelength (405 nm) has been developed to promote practical use of an optical disk having a capacity of 27 GB on single side.
Multilayer recording, multivalue recording, super resolution recording, etc. have been developed as techniques for increasing the capacity of an optical disk. Among the next generation techniques for increasing the capacity, multilayer recording is the most leading technique.
The multilayer recoding, however, has a problem that a very high power laser is required because light is absorbed to an optical recording layer present in the front with respect to a direction of incidence of the light so that intensity of the light reaching a recording layer present in the rear is lowered. In addition, the multilayer recording has another problem that crosstalk occurs easily because information is detected as a signal from other layers than a layer currently used for reading information.
In order to solve the problems, JP-A-2002-15464 has disclosed a method for improving transmittance by provision of a semi-transparent reflecting layer, and JP-A-2002-117585 has disclosed a method for decreasing crosstalk from other layers than a layer currently used for reading information by multiplexing of wavelength and provision of a filter in front of each recording layer.